The present invention relates to a parallel transmission system for transmitting pulses distributed in parallel.
Conventionally, differences in delay times in transmission lines when transmitting pulses in parallel through the lines have been a problem. In order to solve this problem, various systems have been studied. One system is a so-called clock sharing system wherein pulses are regenerated using an average value of phases derived from the timing information obtained from transmission lines (Hitachi Journal Vol. 47, No. 3 (1965), pp. 102 to 113 "Time Division Multiplex PCM Communication System by Pulse Distribution Transmission").
However, as the bit rate becomes high, a difference in delay causes a more adverse effect, resulting in a degradation of waveform margin for pulse regeneration, an increase in bit error rates, or the like.
Such a situation will be described with reference to FIG. 3. In FIG. 3, three pulse trains are simultaneously transmitted from transmitters 21a to 21c over transmission lines 22a to 22c to receivers 23a to 23c. The pulse trains are received at the receivers 23a to 23c at different times because of the presence of different propagation delays in the transmission lines 22a to 22c, as shown in waveforms (a) to (c) in FIG. 3. In such a case, the received pulses are sampled by a clock pulse (d) to align them at a smae pulse regeneration time as shown by a dotted-line. However, this becomes difficult when pulse transmission is performed at higher bit rates.
For waveforms (a') to (c') at a bit rate two times as high as that of the waveforms (a) to (c), i.e., with a pulse width half that of the latter waveforms, it now becomes impossible to share a sampling pulse.
In order to solve the above problem, another method may be used wherein a signal is converted into a CMI (Coded Mark Inversion) code as shown in FIG. 4(a). Coding rule violation (indicated by an arrow in FIG. 4(b)) is applied to each signal at the sending side at the same time. The receiving side detects this coding rule violation to align or make equal the occurrence times of such violation at each signal by using buffer memories. With this method, however, a twofold bandwidth becomes necessary so that the circuit performance or the like may often hinder a high speed transmission and hence degrade signal regeneration.
For avoiding the use of a broad bandwidth, a method of inserting frame pulses 50 at constant intervals as illustrated in FIG. 5 is known. With this method, circuits for discriminating frame pulses 50 and information pulses are required, thus leading to a large size circuit arrangement.